1. Field of the Invention
The invention relates to a method for coding video data comprising at least one intracoded picture (I-Picture), at least one predicted picture (P-Picture) and at least one bidirectional predicted picture (B-Picture), wherein in a first step a P-Picture is predicted from a I- or P-Picture respectively, which is preceding said B-Picture in the order of display of said pictures, and wherein a B-Picture is predicted from said P-Picture of the first step.
2. Description of the Related Art
Compression methods are used for storing and transmitting high quantity of data obtained e.g. with video and audio data. For video processing, the JPEG-Standard is defined to compress a single picture and the MPEG-Standard is defined to compress motion pictures. The Motion-Picture-Expert-Group had defined two MPEG-Standards until now, MPEG-1 and MPEG-2. The compression methods are basing on the following methods: Motion compensation, Discrete Cosine Transformation (DCT), Quantization and Huffman-Coding. A summary of said standards can be found e.g. in: "Der MPEG-2-Standard" from D. Teichner in "Fernseh- und Kinotechnik", 48.sup.th year, No. 5, 1994, pages 227 to 237. The standard itself is described in "Coding of moving picture and associated audio for digital storage media", ISO/IEC DIS 11172, Beuth-Verlag, Berlin.
Successive pictures are very similar in moving sequences of pictures, and that similarity is used in the MPEG-Method. Picture elements will be continuously extending from one picture to the other, except for abrupt changes of scenes. Then, the motion of the picture elements can be transmitted by shift vectors without transmitting the picture data itself. In practice, blocks of pixel, so called macroblocks, of a size of 16.times.16 pixels are used for the motion compensation. Afterwards, the difference of the real macroblock and the shifted macroblock is calculated and the difference is coded and transmitted together with the shift vector. The smaller the difference between the shifted and the real macroblocks is, the more effective the method.
In "Komprimierte Bewegung" of J. Buck, mc, April 1994, pages 114 to 123, the common coding method is set forth. That article describes the three types of pictures used in the MPEG-Standard, namely I-Picture (intra), P-Picture (predicted) and B-Picture (bidirectional predicted). An intracoded picture represents the information of one digital picture. Motion compensation is not processed. The I-pictures are the pictures of origin for processing P- and B-pictures. The P-pictures are derived from pictures of I- or P-type which are preceding in time using a motion estimation. The P-pictures are used as reference pictures for processing shift vectors as well. The B-pictures are deduced from preceding as well as succeeding I- and P-pictures respectively. That is why the B-pictures can be compressed to a greater extent with a constant picture quality. They cannot be used for processing shift vectors and therefore, in contrast to P-pictures, the B-pictures do not contribute to error reproduction. The missing error reproduction allows coding the B-pictures using a lower number of bits, compared to coding the P-pictures, for regulating the bit stream.
The processing of B- and P-pictures is well known in the state of the art. For processing the B-pictures common methods for prediction are used. The pictures are displayed e.g. in the order:
I.sub.0 -B.sub.1 -B.sub.2 -P.sub.3 -B.sub.4 -B.sub.5 -P.sub.6. PA0 I.sub.0 -P.sub.3 -B.sub.1 -B.sub.2 -P.sub.6 -B.sub.4 -B.sub.5.
Therefore, the following order of transmission of the pictures is required:
Thus, at first a P-picture (P.sub.3) is derived by motion estimation from the first I-picture (I.sub.0). Next, the first and second B-picture (B.sub.1 +B.sub.2) is derived from the I-picture (I.sub.0) and the first P-picture (P.sub.3). And from the first and second P-picture (P.sub.3 +P.sub.6) the third and fourth B-picture (B.sub.4 +B.sub.5) is processed.
The main features of picture coding using the MPEG-2-standard are described in "Fernseh- und Kino-Technik", No. 3, 1994, pages 99 to 107, and No. 5, 1994, pages 227 to 237 as well. It is pointed out that video data, which are arriving at the decoder input in the order I-B-P-B-P, are resorted in the order I-P-B-P-B for bidirectional prediction. Afterwards the bidirectional prediction of the B-pictures is processed in one step each from the preceding picture (I or P) and the succeeding reference picture (P).
In U.S. Pat. No. 5,329,365 processing of further B-pictures in addition to said common I-, P- and P-pictures is proposed for improving the picture quality using a bidirectional prediction from an I- or P-picture and a B-picture. However, increased memory would then be required.
Processing of the motion compensation is very costly, so that efficient circuits are required for realtime coding. Nevertheless, the length of the shift vectors used for the motion estimation in the encoding process has to be limited to a relatively small search area for limiting the calculation and memory bandwith expenditure . It is thus impossible to register fast and large movements of scenes in pictures. Candidate vectors are used for extending the search area having a justifiable calculation expenditure, which shift the search area using an offset in relation to the position of a specimen macroblock. The quality of the compression decisively depends on the quality of the method for determining the candidate vectors.
For determining candidate vectors, recursive methods are well known from the literature wherein motion information which is determined for time preceding pictures is used for finding the candidate vectors. Prior to the present invention, such a recursive method would be applied in conventional methods, e.g. a P-picture receives candidate vectors from a preceding P-picture in a recursive manner.